1. Field of the Invention
The present invention relates to an organopolysiloxane composition that can be vulcanized at room temperature into an elastomer that is crosslinked by polycondensation and that does not contain alkyltin-based catalysts which exhibit toxicity problems.
The invention also relates to novel polycondensation catalysts in silicone chemistry, and to the uses thereof as catalysts for the polycondensation reaction of organopolysiloxanes.
2. Description of Related Art
Elastomer formulations that crosslink via polycondensation generally involve a silicone oil, generally a polydimethylsiloxane, with hydroxyl end groups, optionally prefunctionalized by a silane so as to have alkoxy ends, a crosslinker, a polycondensation catalyst, conventionally a tin salt or an alkyl titanate, a reinforcing filler and other optional additives such as bulking fillers, adhesion promoters, colorants, biocidal agents, etc.
These room-temperature vulcanizable (RTV) organopolysiloxane compositions are well known and are classified into 2 different groups: single-component compositions (RTV-2) and two-component compositions (RTV-1).
During crosslinking, water (either provided by atmospheric moisture in the case of RTV-1 compositions, or introduced into one part of the composition in the case of RTV-2 compositions) enables the polycondensation reaction, which results in the formation of the elastomeric network.
Generally, single-component (RTV-1) compositions crosslink when they are exposed to moisture from the air, that is to say that they cannot crosslink in an enclosed medium. For example, the single-component silicone compositions used as sealants or cold-setting adhesives follow a mechanism of hydrolysis of reactive functional groups of the acetoxysilane, ketiminoxysilane, alkoxysilane, etc. type, followed by condensation reactions between the silanol groups formed and other residual reactive functional groups. The hydrolysis is generally carried out by virtue of water vapor which diffuses into the material from the surface exposed to the atmosphere. Generally, the kinetics of the polycondensation reactions is extremely slow; these reactions are therefore catalyzed by a suitable catalyst. As catalysts which are used, use is most often made of catalysts based on tin, titanium, an amine or compositions of these catalysts. Catalysts based on tin (cf. in particular FR-A-2 557 582) and on titanium (cf. in particular FR-A-2 786 497) are catalysts that are very effective.
As regards two-component compositions, they are sold and stored in the form of two components, a first component containing the base polymer materials and the second component containing the catalyst. The two components are mixed at the moment of use and the mixture crosslinks in the form of a relatively hard elastomer. These two-component compositions are well known and are described, in particular, in the book by Walter Noll “Chemistry and Technology of Silicones” 1968, 2nd Edition, on pages 395 to 398. These compositions essentially comprise 4 different ingredients:                a reactive α,ω-dihydroxydiorganopolysiloxane polymer,        a crosslinking agent, generally a silicate or a polysilicate,        a tin catalyst, and        water.        
Usually, the condensation catalyst is based on an organic tin compound. Indeed, many tin-based catalysts have already been proposed as crosslinking catalysts for these RTV-2 compositions. The most widely used compounds are alkyltin carboxylates such as tributyltin monooleate or dialkyltin dicarboxylates such as dibutyltin dilaurate, dibutyltin diacetate or dimethyltin dilaurate (see the book by Noll “Chemistry and Technology of silicones” page 337, Academic Press, 1968—2nd Edition or patents EP 147 323 or EP 235 049).
However, the alkyltin-based catalysts, although very effective, usually colorless, liquid and soluble in silicone oils, have the drawback of being toxic (CMR2 toxic for reproduction).
Titanium-based catalysts, also widely used in RTV-1 compositions, have however a major drawback: they have slower kinetics than tin-based catalysts. Furthermore, these catalysts cannot be used in RTV-2 compositions due to gelling problems.
Other catalysts are sometimes mentioned, such as catalysts based on zinc, zirconium or aluminum, but they have only experienced minor industrial development due to their mediocre effectiveness.
For sustainable development, it therefore appears necessary to develop nontoxic catalysts for the polycondensation reaction of organopolysiloxanes.
Another important aspect for a catalyst of the polycondensation reaction of organopolysiloxanes is the pot life, that is to say the time during which the composition can be used after mixing without curing. This time must be long enough to allow it to be used, but short enough to obtain a moulded article that can be handled at the latest a few minutes or a few hours after it has been manufactured. The catalyst must thus make it possible to obtain a good compromise between the pot life of the catalyzed mixture and the time at the end of which the molded article can be handled (these times depend on the targeted application such as, for example, the molding or manufacture of seals). In addition, the catalyst must confer, on the catalyzed mixture, a spreading time which does not vary as a function of the storage time.